There isn't one. I haven't been very happy with anything I've tried, commercial or open source. I've used Quicken for a while and been fairly happy with the user experience, but I hate the idea of their sunset policy (forced upgrades) and using proprietary format for the data files. Note that I wouldn't mind using proprietary and/or commercial software if it used a format that allowed me to easily migrate to another application. And no, QIF/OFX/CSV doesn't count.
What I've found works well for me is to use Mint.com for pulling transactions from my accounts and categorizing them. I then export the transaction history as a CSV file and convert it to QIF/OFX using csv2ofx, and then import the resulting file into GNUCash. The hardest part is using categories (Mint.com) and accounts (GnuCash) properly. Not perfect by any means, but certainly better than manually exporting transactions from each account.